Which mod is that/what mods are installed and is that the newest build from CurseForge?

I'm using the newest build and this armor is from mythic metals.

Just tested it and it works. So either not the newest build (2.0.0-alpha6) or some other mod is messing with your player model.

I'm not able to reproduce this which is weird because I didn't remove any mods. Closing for now.

Reopen when you can reproduce it, my guess is that it was the broken build with optifine that caused that.